Although its cool using whats essentially flipnote on windows, i think this would be excellent on mobile! my only suggestion is that when adjusting the eraser size to erase a bigger area, it changes my pen size as well, so if you don't remember what your pen size was and you change your eraser size frequently like me- then it can be kind of a hassle. Might i suggest thinking about adding individual pen sizes? so instead of just "size" it would look something like "eraser size" , "pen size" ect. Other than that, seems to be working great! (ofc there's no undo button but i know that's being worked on and for future release) Great job recreating the old retro feel to it too, it has everything you really need to do basic animation without having a lot of fancy tools, keep up the good work! :)